a free flow flat plate solar still (boutebila 1991)

7/25/2019 A Free Flow Flat Plate Solar Still (Boutebila 1991)

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https://dspace.lboro.ac.uk/2134/10424


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This item was submitted to Loughborough University as an MPhil thesis bythe author and is made available in the Institutional Repository

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3/155

\

. l ~ U G H B ~ O ~ ~ ~ ' ~ ~ ~ \ ~ . .

UNIVERSITY

OF

TECHNOLOGY .

.

LIBRARY

AUTHOR/FILING TITLE

C > 1 3 V ~ I ' - A

H

-- - - - - --

-

- - - - -

--

-

ACCESSION/COPY NO.

< ? ~

~ - ~ ~ ? I ? - ~ - - - - - - - - - -

--

--

VOL. NO. CLASS MARK

J (/

J

i.o IV 1995

28 ]UM

1996

:

:

JUl 995

.

O A N ~ - ~ K ~

-

M R 992

U

J L E L ' 5 ~ U i O

- 1 JUL

199 t

,

I

-

I ~ ; I

_.-

\

........ - , , ~ .'(


4/155


5/155

A FREE LOW

FL T PL TE SOL R

STILL

by

HICHEM

BOUTEBIL

A Master s Thesis submitted

in

partial fulfilment

of the requirements for the award of

Master of

Philosophy

of

the

Loughborough

University

of Technology

February

987

Supervisor: MR

Leeson

Department of

Mechanical

Engienering

by Hichem Boutebila


6/155

,,.:

~ ~ . ' . f ~ J

...

. ';''-'' : \.Jbrory

1 i o ~ . = J ~ 1 ] . _

{


7/155

-;, .

I\.

1 , . ,

2J (u f'1LYLf. n I . r r:rr s

Lp:

S 1 S it re

i K ~ fEt.tlp: r LI nb HI red RTf r

iAAl:lHf hlT'o-HTPC i L rdt STr s tf:' E"S

.


8/155

I

wish to

express my

grat i tude

to

my

supervisor r M R Leeson whose

guidance

and

encouragement made t

possible for

me

to complete

th i s

work.

y

gra t i tude

i s

a lso

expressed

to

my

Direc tor

o f Research

Mr T

Davies for his assistance during the

research.

I am gra te fu l to Mr Brian Mace

whose

help in designing and

const ruct ing the solar s t i l l was invaluable

and to

Mrs

Janet

Smith

who typed th i s

thesis.

y

grateful thanks

go

to the

s taf f

and students of the

Department

of

Mechanical

Engineer ing

and

to

l l

my

fr iends

whose

help

and

encouragement were valuable to my work.

I am indebted to the

Alger ian

Government

for providing

me with the

opportunity

to c r ry

out th is research

and

for providing

the finance.


9/155

ii

Solar d i s t i l l a t i o n desal inat ion) of s a l t water i s su i t ab le

for

supplying

water

for

drinking

and agr i cu l tu ra l purposes

to smal l

=mmunities

where

the supply of f resh water i s inadequate

or

of poor

qual i ty nd where solar radiation i s abundant. Historical reviews nd

t heore t i ca l developments of

so la r

d i s t i l l a t i on ,

including

the

phys i ca l

and t e c hn i c a l

r e s u l t s

o f t he va r i ous

des igns

and

=nfigurations

are

reported.

This

research

i s

confined

mainly

to

one

type of

so la r

s t i l l , an

inc l ined

f ree f low

f l a t pla t e

so la r

s t i l l . To s tudy the e f f e c t o f

signi f icant

parameters

a

mathematical

two

dimensional flow analysis

was carried

out

based

on =ntinui ty ,

momentum

nd

energy

equations

for l i qu i d and vapour

flow.

t i s presented together wi th an

i t e ra t ive

=mputational pr=edure.

dimensional

se t of

equations

i s

developed

nd

solved

by

the

Runge-Kutta

method.

I t

i s

slx >wn

tha t

the

Signif icant parameters of

the

=mbined

two ph se flow are the

fi lm

thiclmess

the l iquid

flow rate the =l l e c t o r

length nd

inclination

nd

the

solar radiation.

smal l

sca le f ree

flow

f l a t

pla t e so la r still

was designed and

=nstruc ted nd experimental studies conducted

under

laboratory nd

direct solar nd i

ions

to invest igate features

which

would

seem

to

affect

the s t i l l

performance such as solar radiation wind velOCity

ambient a i r temperature l iquid flow

ra te

nd

angle

of inclination.

Fina l ly the t heore t i ca l

and

experimental re su l t s are combined

together

to

form

a

basis for the design of

a

long large solar s t i l l

for

further

study.


10/155

AckncMledgements

Sumnary

List

of Figures

List

of Tables

Ncmenclature

T BLE OF a:Nl ENI S

INTRODUcrION

Introduction

CllAPI ER 1:

1.1

1.2

1.3

Human Population

and

Energy Demand

1.4

1.5

CllAPI ER

2:

2.1

2.2

2.3

2.4

nergy

Sources

1.3.1

Coal

1.3.2 i l

1.3.3 Natural Gas

1.3.4

Nuclear Power

The Sun

and

Solar Energy

References

Solar

desalination

Introduction

Basin type Solar

St i l l

History

of

Solar Desalination

2.3.1

Algeria

2.3.2 Australia

2.3.3

Orile

2.3.4 Egypt

2.3.5 Greece

.

2.3.6 India

2.3.7 Spain

2.3.8 Tunisia

2.3.9

The US

2.3.10 USSR

Results

2.4.1 Effect of

Atmospheric

Parameters

2.4.2 Design Effects

Page No

i

v

x

x

1

1

1

3

3

5

5

6

6

10

11

11

12

12

16

16

16

18

18

18

18

19

19

20

20

22

22


11/155

2 5

2 6

O API'ER 3:

3 1

3 2

3 3

O API'ER

4:

4 1

4 2

4 3

4 4

4 5

iv

2 4 3 Operational Techniques

COst

of Product Water by Solar

t i l l s

References

SOL R DISTILL TION

GENER L THEDRY

Introduction

Theory

3 2 1 Heat

Balance

on the

Absorber

and

Cover Assembly

3 2 2 Heat

Balance

on

the

Absorber

3 2 3 Heat Balance on the Cover

3 2 4 Heat

Flux by Radiation qr

3 2 5

Heat

Flux by Convection

3 2 6

Heat

Flux by Evaporation

3 2 7 Heat

Lcsses qLc

References

A

FREE

FLOW

FL T PL TE SOL R COLLECTOR

THEORETIC L

MODEL

Introduction

Theoretical Model Development

4 2 1 General

Equations

4 2 2 Boundal:y Conditions

Simplification Process

4 3 1

Liquid

and Interface Phases

4 3 2 Solution of the Liquid

Equations

4 3 3 Vapour Phase

4 3 4 Stream Function Approach

4 3 5

Computational

Procedure

4 3 5 1 Method of Solution

Theoretical

Results and Discussion

References

Page No

25

25

26

31

31

33

33

34

34

34

35

35

35

39

40

4

42

42

45

46

46

47

49

50

52

52

54

76


12/155

0iAPI ER 5:

5 1

5 2

5 3

5 4

5 5

5 6

5 7

0iAPI ER

6:

6 1

6 2

6 3

6 4

0iAPI ER 7:

7 1

7 2

v

EXPERll1ENTS ND INSTRUMENT TION

Introduction

t i l l Cbnstruction

5 2 1 The t i l l

5 2 2 The

Tank

5 2 3 The Pump

5 . 2 4

fubes

Outdoor

t i l l

The Laboratory t i l l

Instrumentation

5 5 1

Temperature

Measurements

5 5 2

Solarimeter

5 5 3

Data Logger

5 5 4 Liquid Flow Rate Measurements

Tests

5 6 1 The Outdoor t i l l Tests

5 6 1 1 Principles

5 6 1 2

Tests

5 6 2 Laboratory Tests

5 6 2 1

Principles

5 6 2 2 Tests

References

EXPERIMENTAL RESULTS

Introduction

Laboratory

Results

The Outdoor Tests

Cbnclusions

CONCLUSIONS ND SUGGESTIONS FOR FURTHER

WORK

onclusions

Further

Work: a Long

Large

Scale Solar

t i l l

Plant

Page No

78

78

79

79

79

79

81

81

81

87

87

87

88

90

90

90

90

94

94

94

95

96

97

97

97

99

116

117

117

119


13/155

APPENDICES :

ppendix A l

ppendix 1\2:

ppendix A3:

ppendix A4:

v i

Relative Scale Values

Simplif icat ion

Process

Tabulation o Velcx::ity Vectors

A4 1

Nag Rout ine Programme D 2 BAF to

Solve 2F' + FF ;

A4 2 The

Mathematical

M ldel

Programme

Page

No

123

125

130

131

132


14/155

Q IA Pl ER

1:

Figure 1.1

Figure 1.2

Figure

1.3

Figure 1.4

Figure 1.5

Q IA P l E R

2:

Figure 2.1

Figure

2.2

Figure 2.3

Figure

2.4

Figure

2.5

Q IA P l E R

3:

Figure 3.1

Figure 3.2

Figure

3.3

Figure 3.4

vii

LIST OF FIGUR S

Conversion of energy

from

one

form to

another

GrcMth of world population 1400 2000

Estimated world energy demand 1800-2000

The electranagnetic spectrum

Average

annual so la r r ad ia t ion

horizontal

surface

a t

the

ground

on a

Different

types of

solar s t i l l s used around

the world

Basin

type

solar

s t i l l

Tilted

s t i l l

Inflated

plastic

s t i l l

Geographical

locat ions of the s t i l l s in

North Africa a t

the

end of 1957

Diagrammatic sections of solar s t i l l showing

s igni f icant

energy t ransport

streams

to,

Page No

2

4

4

7

9

13

14

14

17

17

from and

within

the

s t i l l

32

Evaporat ive heat t r ans fe r

qe

vs

cover

temperature

Tg

for

different

values of

brine

temperature Tw .

Cover heat

loss vs

ver temperature

g

for

values of

ambient temperature

Ta

and

wind

ve loc i ty

O1aracteristic char t

for

thermal performance

37

37

of a

solar

s t i l l

. . . . . . . . . . . . . .

38


15/155

Qi l \P l ER 4:

Figure 4.1

Figure

4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure

4.6

Figure 4.7

Figure

4.8

v i i i

Solar = l l e c t o r

A

two-dimens iona l vapour behav i our in

re la t ion to Hquid-vapour in terface

DiInensionless longi tudinal vap:>ur v e l = i

y

DiInensionless

t ransversa l

vap:>ur v e l = i y

Variat ion o f

dF/dB

with u t

Var ia t ion

o f

l i qu i d

ve loc i ty

with f i lm

thickness

for

various

i nc l ina t ions

Var ia t ion o f

the

l iqu id th ickness

wi th

the

= l l ec t o r length

Film thickness var ia t ion with heat flux

for

various incl inat ions

Figure 4.9a EV versus Y

Lr


16/155

CliAPl ER

6:

Figure 6.1

Figure

6.2

Figure 6.3

Figure 6.4:

Figure

6.5

Figure 6.6

Figure 6.7

Figure

6.8

Figure

6.9

Figure

6.10

Figure

6.11

Figure 6.12

Figure 6.13

Figure

6.14

Figure 6.15

CliAPl ER

7:

Figure

7.1

Figure 7.2

e

Water

production versus

pl te

temperature

Water

production

versus

l iqu id flow r te

Water production versus s t i l l

inc l ina t ion

Daily s t i l l

output

Average

dai ly insolat ion

Average d i ly wind speed

Average

d i ly

ambient i r temperature

Environmental data of

day

1


day

2

Environmental

data

of

day 3

Environmental

data

of

day

4


day

5

Environmental

data

of

day

6

Environmental data of day 7

Environmental

data of day 8

A

long large

scale sol r s t i l l

Evaporated water

of a

long

scale sol r s t i l l

Page

No

98

100

101

104

105

106

107

108

109

110

111

112

113

114

115

121

122


17/155

0IAPl ER 2:

Table

2 1

Table 2 2:

AI PFNDIX A3:

Table

Al:

Table A :

LIST

OF

TABLES

Data on the most

impor t an t

so l a r

dis t i l l a t ion

plants tha t have been bui l t

from 1872

to

1980

.

Lis t of

the s t i l l components

tha t have

proved to be reasonably sa t i s fac tory

in

solar s t i l l s around

the r l d .

Tabulation

of

l iquid velocities

Tabulation

of vapour

velocities

Page

No

21

24

130

130


18/155

x i

A Film th ickness

a* Dimensionless fi lm thickness = A/YLr

2

Br Brinkman

number =

J

U

r

/

A 1

Tr

Cp

Heat

capaci ty

Fe External forces

9

Gravitat ional acceleration

I Ehthalpy

l\..

Kutateladze

number

= f :, TLr/L

L Heat of vaporization

M )Jc;I)JL

m Variation of

viscosi ty

m Mass

t r nsfer

per

unit area nd per uni t

of

t ime

-

n

Normal uni t

vector

P

Pressure

p*

Dimensionless pressure

m Dimensionless m::xlified

pressure

=

P-P

r

)/6P

r

Pe

Pr

Jw

Re

t

T

-

t

U

-

V

V

Peclet

number

=

U

r

YrP Cp/A)

Prandtl number =

)JCp/A

Plate heat flux

Reynolds number = U

r

YrP/)J

Time

Temperature

Tangential uni t vector

Longitudinal

veloc i ty

Dimensionless longitudinal veloci ty

= U/Ur

Dimensionless

interf ci l

longitudinal

veloc i ty

difference

UGi - Vr,i)

= ----"'=-,Z\crO,------:=-

r

Velocity vector

Transversal

veloc i ty

Dimensionless

t ransversal

veloci ty

V/V

r

Dimensionless

i n t e r f c i l

t ransversal velocity difference

VGi

- VLi)

= - - - - ' - ' - - - . , 6 . V T r ~ ~


19/155

x

We

X

2

Weber number = J/PL U

r

Y

Lr

x

Y

Longitudinal

CXJOrdinate

Dimensionless longi tudinal CXJOrdinate = X ~

Transversal CXJOrdinate

y

Dimensionless t ransversal

CXJOrdinate =

Y/Yr

Greek symbols:

i

Variat ion o f the densi ty

y pg/Pr.

r

Viscous

s t r e s s tensor

P Density

f

Stress

tensor

Dynamic viscosi ty

A

Thennal conductiv i y

8

Dimensionless

temperature = T-T

r

)/6T

lj Stream funct ion

n Angle o f inc l ina t ion of

the pla te

to the

oorizontal

Indices:

G

Vapour

i

nter face

L Liquid

P

Par t ic le

r Scale

S

Saturat ion

W Pla te

Z G,L)

Dimensionless

term


20/155

What

i s

energy?

I s

it f lash of l igh t? A burs t of heat?

Not

rea l ly . These are j u s t

two

among many

forms

o f

energy:

e l e c t r i c

chemical biochemical nuclear kinetic gravitational magnetic and

forms

we

have

not

ye t

discovered.

That

i s

why

the re

can

never

be

a

t rue

energy

shortage. As Einste in demonstrated in hi s

famous

formula

E =

2

,

tha t

everything

in the

Universe i s

energy.

Light

heat

matter

-

i s j u s t energy

in

one form

or i n

t r an s i t between

different .forms.

Figure 1.1 shows

the

conversion

of energy from one

form

to arnther.

nergy surrounds us in

inconceivably

vast quanti t ies .

However, while

the ear th

i t s e l f

i s

composed

o f

so

much

energy

t ha t

we

can

never

complain about i t

we

are

still

concerned about

harnessing

energy

supplies

mainly

in the 20th and 21st centuries

which

i s due

to

ever

increasing energy demand. This increas ing demand i s due to human

populat ion

growth

and r i s i ng

indus t r i a l i za t ion

and s tandards o f

l iving.

1 2 lM\N roPULATICN AND

ENERGY

DEMI\NI

The

ut i l i sa t ion

of

power

by man

in

the

past

followed a s imilar

t rend

to

the growth of the population.

Figure 1.2

shows

some estimates

of

the

t o t a l

world popula t ion

with pro jec t ions to

the end o f

t h i s

century

Brinkworth

[ ]

As

can be seen from Figure 1.2, up to

the

19th

century the

t o t a l

populat ion

had

remained

in

the i n t e rva l o f

300

to 1000

million. The

ra te of

increase in

that

period

was

nearly

zero

to

0.75

per

year.

The

big

change

occurred

in

the

20th

century

passing

from

approximately

1500 mil l ion a t the

beginning

of the


21/155

nuclear e n ~ r g )

radiant

n e r ~ r

/ - ~ - - - - - - - - - - - - - - - - - - - - - - ~ ~ ~ ~ ~

V

electrical enerGY

c n c q ~ 1

FIGURE

1 1 :

ONVERSION

OF ENERGY FROM ONE FORM

TO

NOTHER


22/155

3

century to about 5000

mil l ion

a t the end of i t , with

a

ra te increase

o f 2

per

year .

The

curve

for energy demand steepens

more

rapidly than tha t fo r t he

world population see Figure 1.3), mainly from the mid-20th century

where

the ra te increase

was

higher

than

5

per

year. This was

caused

by t he ext raord ina ry

technology

development

and

soc ia l changes ,

espec ia l ly

a f t e r the

Second

World

War.

This

development

has

dramat ica l ly

inc reased t he demand fo r energy

to

the ex t en t o f

reaching

a

c r i t i ca l s ta te .

The

future

dem8f .ds

for

energy

are

l ike ly

to

go

up

both

on

account

o f

increasing

population and owing to

a bet te r

standard o f l iv ing in

a l l

parts

of the World.

What, therefore,

are

the

energy al ternat ives to

meet these

demands?

To

answer th i s

quest ion

a br i e f discussion

on energy sources

wil l be

given.

1 . 3

ENERGY SOORCES

There are two types

o f

energy

sources: those which

are exhaust ible

c l a s s i f i ed as non- renewable such as

fo s s i l

fue l s

(Le . coal ,

o i l ,

gas) ,

nuc lea r fue l s (Le . uranium)

and

geothermal power; and the

inexhaust ible sources

c l a s s i f i ed as renewable

l i k e so l a r , wind and

hydro power.

Today man

re l i es

on

f ive

main sources of energy. The

foss i l fue ls -

coal , o i l

and na tu ra l

gas

-

account fo r no l e s s

t han 95

o f wor ld

wide consumption, the remainder coming from hydroelectr ic with

3

and

nuclear power

s ta t ions

with

2 ,

Garg [2].

1.3 .1 bal

Coal,

which i s

a

combustible

sedimentary rock

formed

from

the

remains

o f p lan t l i f e , s

the

most p len t i fu l o f

the

e a r t h s fo s s i l fue l s .


23/155

4

6

I

I

I

5

E

04

x

.c

~

3

a.

a.

:g2

1800 1900

2

year A D

FIGURE 1 2 :

GROWTH OF WORLD

POPULATION

1400-2000

Afte r 1)

50

40

18

19

year A.D.

I

I

I

I

I

2000

FIGURE

1 3 :

ESTIMATED

WORLD

ENERGY

DEMAND,

1800-2000

After 1)


24/155

5

Ninety three

per cent of

CDal

reseIVes

are concentrated

in

only three

countr ies the

USSR,

USA

and China

[2] . This

uneven dispar i ty

in

coal reserve di s t r ibu t ions add

to

t ha t the pol lu t ion which can be

caused

by the re lease of

carbon

dioxide through combustion and

the

r i s k o f e x h a u s t i b i l i t y

in

t he

nea r

fu tu re

make coa l no t

recommendable as a solution for the 21st century.

1.3.2

i l

Oil,

which

i s fonned from

marine

l i f e by the de=mposition

of

living

matter ,

i s

the

most used energy

source

in

the

world.

While

coal

i s

concentra ted in c e r t a in places, o i l i s widely dis t r ibu ted in

the

world

The

exploration of

oi l

which i s

also exhaustible and

causes

atmospheric pollution, i s

more uncertain

than tha t

for

coal and t i s

bel ieved t ha t o i l production wi l l reach i t s peak in the 1990's and

s t a r t to

decl ine a f t e r tha t .

That

i s

why the re are

growing

d i f f i c u l t i e s in mainta in ing an equi l ibr ium i n o i l demand.

Add

to

these disadvantages, the

pol i t i ca l

manoeuvres in o i l

pr ices

which

have

dramatically

worsened

the

s i tuat ion

to

the

extent of

reaching

an

energy cris is .

t can be

sa id then,

from the

actual infonnat ion

that

oi l cannot be

a

solution to meet the energy

demands

in

the

future.

1.3.3

Natural Gas

Natural

gas, which

always

accompanies

crude

oi l

i s

a hydrocarbon

composed essent ia l ly of 90

methane

(0l4).

he

remainder i s ethane,

propane and butane . 80 o f

gas

r e se rve s in t he wor ld are

concentrated

in

a few regions Garg [2]:

Middle

East and Africa: 31 ,

Russia: 39.9 ,

USA: 8.3 .

This uneven

dis t r ibu t ion

of reserves

ensures t ha t na tura l gas wi l l never

become

ava i lab le

to

meet any

generalised world energy

demand


25/155

6

1.3 .4 Nuclear

I\ Jwer

Nuclear power i s one o f

t he

new energy

sources

which

have

been

extens ive ly developed

in

the

l a s t 30 years. The

power can be

obtained

by

two contrasting types of nuclear reaction:

i )

The

fissioning of

heavy

atonic isotopes (Uranitnn 235)

ii

The

fusion of

the

isotopes of

hydrogen

in to heavier

helitnn.

It can

be sa id

tha t it

i s a

promising source o f

energy

to meet

the

energy

demands

but

the

cos t o f

ins ta l l a t ion ,

the

use

of

nuclear

weapons a t

Hiroshima

in

1945)

and

mainly the

pol lu t ion

which

threatens

l i f e on

ear th

the

las t

example

of th i s pollut ion was

the

CheITlObyl

disas ter

in Russia, which

caused

the

death

of many

people

and polluted

eveIything round

the

s i te , reaching

many neighbouring

=untr ies )

are

to the

disadvantage of th is

source of

power.

There i s no

need to

speculate

fa r

beyond the non-renewable

sources

for

each

one has

sh:>wn

a

lack

n

meeting the

demand for

energy

n

the

future.

The

only solution for us

i s to develop the

renewable sources

such as solar , wind, oceans, agr i cu l tu ra l wastes , and solve the

technical problems

which

face them.

Unl ike

the

non-renewable

sources ,

so l a r energy causes no

environmental pollut ion and it i s the only

one

for

which

technology

i s

available n many applications.

1.4 ' HE SUN AND SOLAR ENERGY

The sun i s responsible

for most of our

energy

resources , including

foss i l fuels , so la r ,

wind, hydroe lec t r ic power

and also

food.

In

other words,

without

the

sun t he re would

be no l i f e on ear th .

The

so la r radia t ion which

i s an inexhaus t ib le energy source, comes

to

ear th as

l ight ,

which

i s

a

form

of

electromagnetic

radiation.

Most

solar radiat ion

fa l l s

between

0.15

and 120 m but the

practical

one


26/155

COSMIC

GAMMA

RAYS

RAYS

REACHiNC

.a.ATH

(MITTED

FROM

SKY RADIUM

X

RAYS

_l

1

H'GH

FR.EQUENCY

DSCllL rlONS

PR ODUCED

BY

X - ~ A Y

rUBES

7

WAvELENGTH [MICRONS)

.

,.

lOl.r.7 0

ULT'RA

IN FRARED

VIOjLET

pR.QOUCEO

ay

HE T

0

,

I

I

I

I

I

I I

~

PIIDOUCEO By

I

IElECTlItC LAMPS

I

I I

,

10

RADIO

WAVES

PP QOUCEO BY

HIGH-FAEQUENCY G(NEIlATOR.

ELECTRIC

WAVES

PROOUCED er

ELECflUC

GENERATO S

r - - - - - - - - - - - - - - - - - - - - ~

~ - - - - - - - - - - - - - - - - - - ~ - -

I ,

I I

~ F ~ : : ~ - - ~ ~ - - ~ ; _ I r _ ~ ~ : J ~ ~ r - ~ ~ : = : : ~ ~ ~ ~ ~ ~ ' ' > 4 r r - - - - l r - - - - l r - - - - l r - - - - l :

. l. 100

' ~ ~ RO

MOOLE NEAQ THE [UTI-

60

w

,0

~

l

uo

VISIBLE

SHORT WAVE

INFRARED

DISCQIMINATlO"l

f

HEAT THER Py - DRYING

O ~ ~ ~ ~ ~ ~ ~ ~ ~ I ~ L J ~ ~ L J ~ 2 L ~ U U L ~ ~ ~ ~ ~ = = ~ ~ 1

0.) 0.J.5.

0

0 5 05 055 06

0.65

0-7 0-75 10

l

)0

0

50

FIGURE 1.4 :

WAVELENGTH (MICRONS)

THE ELECTROMAGNETIC

SPECTRUM

Af te r

(3)


27/155

8

f a l l s between

0.15

and ~ m

Figure

1.4 shows

the e lec t romagnet ic

spectrum

Giedt

[3].

The

ear th

receives annual

energy from

the sun amounting to 10

18

kWh

Garg [2] . This i s

equivalent

to

more

than 500 000 bi l l ion barre ls of

o i l o r about 1000

t imes

the energy o f

the

known reserves of o i l o r

more

than

20 000 t imes

the

present annual consumption

of energy o f

the

whole world.

The most favourable s i t e s

for exploiting solar energy

are

oonfined to

areas

between

la t i tudes

35

deg

north

and

south

of

the

Equator which

rece ive some 2000-3500

hours of

sunshine per

year

[2] .

Figure

1.5

shows

the average annual solar

radiat ion on a

horizontal surface a t

the

ground

Sellers

[4].

The

major technical

obstacles

which

face

solar energy are:

i so l a r

energy

s

a

d i f fu se energy form

i .e . ~ l i t o u t

ooncentration

i i the

short

term

varia t ion of solar energy.

hese obstacles

imply things:

i

large

areas

and s tructures are necessary

to

provide t he

needed

energy

i i energy must

be

stored

for

time wh n t i s not available.

Unlike other

sources

o f energy so la r

energy

has several unique

fea tures which

place t in

an advantageous

posi t ion. Most o f

the

materials

required for

making solar

apparatus are

eas i ly ava i lab le

and are not very =mplex t design and solar energy

can

be used for

a

variety of

applications

such as for:

heat ing

water

for

domest ic

i ndus t r i a l

and

agr icul tura l

purposes;


28/155

> :.

FIGURE 1 5 :

AVERAGE ANNUAL

SOLAR RADIATION

ON

A HORIZONTAL

SURFACE

AT

THE GROUND

THE UNITS

ARE

KILO-

LANGLEYS

PER YEAR

VALUES S O w ~

IN PARENTHESES

ARE

IN kWh/rn

2

YEAR

f ter

4)


29/155

1

dxying agr icul tural and indus t r ia l products;

space heat_ing and

cooling;

ref r igerat ion for preservation of

food;

desalination

and d is t i l l a t ion of water;

cooking

of food; and

e lec t r ic i ty production

The appl i ca t ion

o f

s o l a r energy i s wider than any other form and

there fore to ob ta in t echnologica l progress , only a

spec ia l i s ed

applicat ion should be

oonsidered. In

th i s

work we have

addressed

our

a t tent ion to

the

so la r desal inat ion

process.

1.5

REFEREN ES

1

B J

Brinkworth:

Solar

energy

for

man . The

Canpton

Press,

1972.

2

H P

Garg:

Treat ise on s o l a r energy:

Vol. 1,

fundamentals o f

so la r

energy .

A

Wiley-Interscience

Publication,

1982.

3. W H Giedt:

Pr inc ip les

o f enginee r ing

heat

t ransfe r . Van

Nostrand,

New

Jersey,

USA

1961.

4.

W D Sellers:

Physical Climatology . University of Chicago Press,

1965.


30/155

2.1

:INl RaXX:I ICN

11

OIAPI ER

2

SOLAR DESALINATICN

Solar

desa1inat ion

or

d i s t i l l a t i on

of

s a l t

water i s su i t ab le fo r

supplying f resh

water

to

smal l

communit ies where the supply of

potab le water i s inadequate o r o f poor

qua l i ty

and where s o l a r

radia t ion

i s abundant.

' 'he basic approach of

d is t i l l ing

sa l ine water by solar

energy

i s

the

natural hydrologic

cycle which

consists of :

i

The absorpt ion o f

s o l a r

rad ia t ion as hea t

by

oceans,

r ive rs ,

lakes, causes evaporation

of

water;

i i The vapour produced s

t ranspor ted as humidi ty

of

the a i r to

cooler regions by

means

of winds;

i i i hen the

a i r

vapour

mixture

i s cooled, the condensation occurs

and causes

i t s precipitat ion

as ra in

and

S 'DN.

This process

i s motivated by

solar

energy which penetra tes the water

surface, warms t and causes i t s evaporation. The t ransport of the

vapour to the cooler

regions where

t condenseS are caused by winds

which are also produced by solar

energy.

By analogy to this, man has reproduced, on a small scale,

the

natural

cycle. As a resul t the following process by which

pure

water

can

be

produced n a solar

s t i l l

is :

i The

production of

vapour fran the solut ion;

i i The

t ransport

of

th is vapour by convection to

the

t ransparent

cover where

t i s

cooled

and condensed; and

)

The

col lec t ion

o f

the

condensed

water.


31/155

12

There

are

several

so la r

st ll designs which use t h i s

process.

They

may dif fe r

from

one another in shape and mater ials used, but a l l use

the same

pr inciples and

serve the same functions. Figure

2.1 shows

different types

of

solar

s t i l l .

2 .2

BASIN-TYPE

SOL R STILL

The

most

=mmonly

used

solar s t i l l i s

the

basin-type s t i l l

which

i s

also Irnown as a

greenl Duse-type,

r=f- type simple-type. The design

=mprises a

horizontal

blackened

surface

which i s f i t t ed

with

sal ine

o r brackish water in

a

shal low o r deep dish and covered

with a

t ransparen t

s loping

surface

on which

water

can

be

condensed.

The

CXJVer which

can be ei ther glass

or

plast ic, i s sloped towards troughs

where

t i s

col lec ted and

then

stored.

Such a

st ll i s

shown in

Figure 2.2.

In operat ion,

so la r

energy

which

i s

t ransmit ted by

the

cover

i s

absorbed by the solut ion (30 ) and the basin (70 ), ooper [1]. Heat

which i s =nducted from the

black

surface to the solution, ir lcreases

water

temperature

and thereby causes

evaporation.

The t ransparen t

cover which

i s

=o l e r than the brine, condenses the warm air-vapour

mixture

which has

been carried

by

convection

currents. The =ndensed

moisture s l ides down the slope to the =l lec t ing troughs from

which

t passes to storage.

To increase

the

product iv i

ty

above t ha t achieved

in

the-hor izontal

basin

s t i l l

t i l t e d

o r

inc l ined

so la r

s t i l l s

have

been used.

The

reasons

for th i s improvement

are

that the t i l t ed

surfaces

in tercept

more energy

per

square metre of co l l ec tor area

and

t ha t covers

re f lec t less

sunlight because

of

a

more

dire t

angle

of

incidence.

A

t i l t ed s t i l l

i s

i l lus t ra ted in

Figure 2.3.

2 .3

HIS'lORY OF SOL R DES LIN TICN

Severa l

repor ts

and

hi s to r i ca l

reviews

of

so la r

desa l ina t ion are

available

in the

l i t e ra ture

review, Telkes

[2,

3]; Daniels [4]; Howe


32/155

3

BASIN-TYPE PLASTIC COVERS

TILTED

WICK

PREFABRICATED

TRAY

''''

mm

c:c::J

DOUBLE

: TUBE

:=:

EXTERNAL CONDENSING

l ~ ~ i o l ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

olar

Disliliotion

l o n t ~ _

Transparent

Distillate

t

BASIN-TYPE GLASS

COVER

FIGURE 2 1 : DIFFERENT TYPES OF

SOL R

ST LLS USED ROUND

THE

WORLD


33/155

Angle of glass

10-20)

14

Glass cover

FIGURE

2.2:

BASIN

TYPE SOLAR

STILL

Distillate outlet

:::: -

Brine outlet

FIGURE

2.3:

TILTED

STILL

Insulation

Condensate trough and

F e e d

water


34/155

5

[5] , United Nations [6], the t r e a t i s e

o f

Talber t

e t

a l [7] and the

l a tes t

book

by

Malik

e t al

[8].

The ea r l i e s t solar dis t i l la t ion plant on record was the large basin

type

s o l a r still designed in

1872

by

a

Swedish engineer , Car los

Wilson

in Las Sal inas in the

province

o f

Antofagasta

in Northern

Chile .

t

had

a

co l l ec tor area o f 4700

m

2

and produced

20

m

3

o f

d i s t i l l e d

water

per day during the Summer.

t

has been reported,

[2 ] , t h a t t he still

worked

u n t i l

1910,

t h a t is 30 ye a r s

approximately.

A

detai led descripti.on of the design and

operation

o f

th is

f i r s t s t i l l

was

reported

by Harding

[9]

in 1883.

t has been

repor ted

[7]

tha t in the ear ly

1930 s,

a t i l t ed-wick

design had

been proposed by

Trofimov in

Russia.

During the

Second

World War,

a

new i n t e r e s t

in so la r

d i s t i l l a t i o n

emerged

with

the

invent ion

by Dr

Maria

Telkes of i n f l a t ed p l a s t i c

stills to be used in emergency

l i f e

r a f t s of

the US Navy

and Air

Force, [10].

These uni t s

cons i s t ed o f

an

i n f l a t a b l e smal l

p l a s t i c

envelope containing

a black

absorbent pad

made

of

cellulose sponge

to

be sa tu ra ted

with

sea water before

in f la t ion , and a d i s t i l l a t e

co l l ec tor b o t t l e connected to the bottom of the envelope. Vapour

which would be produced by so la r energy on

s t r i k ing

the absorbent

pad, would

ondense

on the plast ic envelope and dr ip in to t he bot t le .

t was

reported

tha t over 200,000 o f these uni ts were

produced

during

World War I . Such

a

still i s

shown

in Figure 2.4. After the war,

she

inves t iga ted

glass-covered

s t i l l s

and

in

1951,

she

designed

a

glass

greenhouse-type

s t i l l .

She

has

also

reported

[3] experiments

on

t i l ted-wick s t i l l s where 20

of

them were constructed in 1960-6l.

During

the decades

fol lowing

World War 11,

sus ta ined

drought

condi t ions

i n many pa r t s

o f the

world caused problems

in water

supply.

The

use

o f

s o l a r desa l ina t ion seemed to

give a

so lu t ion to

t h i s

problem

by

producing

f resh

water.

All

over

the

world,

wOl k

on

so la r d i s t i l l a t i o n began.

Amongst the countries which experimented

with

so la r

desal inat ion

were:


35/155

6

2.3.1

Algeria

In 1953

Cyri l

Gomella developed

various t ray- type so la r

stills

in

Algeria [11].

More

t han twenty s t i l l s of

ten

different designs

were

t e s t ed and

some

o f

them

were so ld

comm ercia lly throughout

North

Africa ,

Senegal, Cyprus and Aust ra l i a . Figure 2.5 shows the

geographical

loca t ions

o f

the s t i l l s

in

North

Afr ica

a t the

end o f

1957. Savornin

and

Lejeune [12] inves t iga ted f ive other types,

including

th ree

t r ays and

one t i l t e d . These

designs

at tempted

to

improve convection within the s t i l l .

2.3.2 Austral ia

In

1953,

the

SIRO

(Commonwealth

Scient i f ic and Industr ial

Research

Organisation) in

Austral ia ,

s tar ted invest igat ing so la r s t i l l s . hey

developed

a uni t

s imi la r t the Gomela s t ray

and

from

963

t 1967,

CSIRO bu i l t

more

than 8 glass-covered s t i l l s . The aim of these

experiments

was t improve the efficiency of so la r s t i l l s by studying

the

e f f e c t o f

some parameter s

such as wind

ve loc i ty ambient

a i r

temperature, cover incl inat ion,

water

depth,

thermal

capacity,

base

and

edge losses;

and

by

operat ing the

stills

under a range o f

conditions with sa l ine water supplies varying from brackish water t

sea

water , Morse

and

Read

[13 and

14].

Also a

va r i e t y of

mate r ia ls

were used

in

s t i l l

construction in an

attempt

t evaluate the i r

l i f e

expectancy and re l iab i l i ty

[15,

16, 17].

2.3.3 0li1e

t was mentioned above t ha t the f i r s t

so la r

still in the world

was

bui l t

in Chile in 1872.

In

1969/70,

two

so la r

s t i l l

pi lot

plants were

bui l t a t

Quilagua

by Santa Maria Technical University [18]. In 1972

a t the Por t

o f

Pisagua,

four inCl ined so la r

stills

were i n s t a l l e d

[19]. The purpose of the work was t make theoret ica l

predict ions

of

s t i l l

character i s t ics

under

different environmental conditions.


36/155

- bl-cJ.:

1 0 .0 .. JNd

t -

P< d

ppori

]

t r ' I U ~ t

...... r o p

..

- .,1 '-


37/155

18

2.3 .4

Egypt

During 1960

several

smal l

so la r s t i l l s were

t es ted

by the National

Research

Cent re Hafez e t a l [19] . It has a l so been

repor ted

[7]

tha t in 1966

a

plas t ic

= ver ed

still

was

developed

and

tes ted

on

the

Red Sea

coast .

2.3.5

reece

From

1964

to 1973 several

la rge

s t i l l s located in di f fe ren t i s lands

were b u i l t fo r a

t o t a l

area o f 28891 m

2

. A V-shaped

cover

made o f

g l a s s

and

p l a s t i c was t r e a t e d w i th

a va r i e ty o f

cons t ruc t ion

mater ia l s . They have

a l so

in t roduced new

concepts i n des ign ing a

l a rge

still to inc rease the da i l y

product ion

Delyannis

e t

a l [20] .

These two concepts

are:

i a s tronger

=ns t ruc t ion of

the = ve r , and

ii making the s t i l l surface

including s ide

walls as a whole area.

2.3.6

India

Five

small t ray- type s t i l l s were =ns t ruc ted

in 1957 by

the

National

Phys ica l

Labora tory

in

New Delhi Khanna e t a l [21]. To

asse s s

the

per formance o f var ious

m ate r i a l s

and g la s s cover des igns , severa l

other experimental so la r

s t i l l s

were tested, Gomkale e t a l [22].

They

a lso s tudied the

ef fec t

of

different

parameters such as

atmospheric

variables

=ns t ruc t ion

mater ia ls

and

operational

techniques

on

the

performance of solar

s t i l l s ,

Ahmed e t a l [23]. Two of the =nclus ions

obtained

were

tha t :

20

degree

incl inat ion was the bes t angle for the

covers

and t h a t t he average e f f i c i e nc y was about 30

o f

energy

ut i l i sa t ion in the so la r s t i l l .

2.3.7

Spain

It

has

been

r epor ted

[7]

t h a t

dur ing

1958

two

smal l

t r a y s o l a r

stills were cons t ruc ted

to s tudy t he

e f f e c t

o f var ious glass


38/155

9

incl inat ions nd construction techniques.

I t was found

that a

glass-

covered st ll

with

a

shal low bas in and

a low inc l ina t ion cover was

the bes t

design.

In 1966, an

869 m

2

st ll

was completed a t

Las

Marinas. The i n s t a l l a t i on

was

des igned to

provide

a vi l l age of 300

persons

with

fresh

water.

2.3.8

' l \mis ia

Since 1962, the so la r

energy group

o f

the

Tunis ian

Atomic

Energy

Authority has

been

act ively studying solar

dis t i l la t ion [24]. More

than a dozen

were bu i l t and

a t the

beginning

o f

1967, th ree la rge

solar

dis t i l l a t ion

sta t ions

were

constructed

[25].

2.3.9 ' be

US

After

the Second World

War,

many

research

cent res

in the

US

amducted work

on

solar

d i s t i l l a t i on . The

Universi ty o f

Cal i fo rn i a

s t a r t e d ts inves t iga t ion in 1952 and

cont inued

fo r more than

20

years. Various configurations

for

simple

solar

s t i l l s were bu i l t nd

tes ted

in

t rying

t

reduce

capi ta l

cos ts

nd

improve

eff iciency.

The

work aimed t

study the

features

which

would

seem t

affect

the s t i l l

efficiency,

such

as various geometrical configurations,

batch-feeding

versus continuous-feeding,

means

of

recircu1ation

of

air ,

nd kinds

nd

thicknesses

of

insulat ion.

I t was concluded that the

conditions

which

seemed

to lead

to maximum eff ic iency are:

i

a low heat capacity of the s t i l l nd the water contained in i t ;

i i a low

incl inat ion of

the vapour-tight

t ransparent

cover;

nd

i i i

good

insulat ion

of the

bottom of

the s t i l l .

A

summary of

th is

work can

be found

in

Howe nd Tleimat [26]. From

1958 to

1965,

the

Office

o f

Saline Water

planned

a

solar dis t i l l a t ion

programme nd financed the Bat tel le Mem=ia1 Inst i tu te to

build

nd

t e s t

severa l

types of s o l a r

st lls

a t Daytona Beach Sta t ion in

Florida

[27].

Many

other

unive rs i t i e s

and

research cent res

in

the

US invest igated

solar dis t i l la t ion .


39/155

20

2.3.10

USSR

I t has been reported

[7]

tha t during 1956 the Solar nergy Laboratory

in

Krzhizharovsky

in

Moscow began

invest igat ing

so la r

s t i l l s

as

a

means

o f

supplyirq water to ar id and

semi-ar id

lands

in

Russia.

In

1962,

a so la r s t i l l was designed and t e s ted a t Tashkent University.

From 1961 to 1965, experimental s t i l l s

were

t es t ed in Turkemenian and

based on th i s work, =nst ruc t ion of a large s t i l l began in Ashkhabad

in 1969

[28] .

t has been r e p o r t e d

i n

t he

l i t e r a t u r e

rev iews t h a t s o l a r

d is t i l l a t ion

has

also

been

invest igated

in

the

following

= m t r i e s :

I t a ly Japan, Taiwan, South Afr ica , Libya, France, Morocco,

Kenya,

ew

Caledonia, West

Indies,

Pakistan,

Cyprus, Iran,

Senegal,

Mexi=,

China etc.

Data on

the

most

important

solar d is t i l l a t ion plants

tha t have been

bu i l t

fIOm

1872 to 1980 are shown in Table 2.1 [29].

2 .4

RESULTS

Here

are

the

conclus ions obtained in

a

carefu l s tudy o f

the

s ign i f i can t r e s u l t s

presented

by

many inves t iga to rs a l l over the

world.

Solar d is t i l l a t ion should be =nsidered a

possible

method for water

supply under

the

following

circumstances:

1. Natural fresh water

i s not available

and sa l ine or brackish water

i s available;

2. The

cl imate i s

good i .e.

solar radiat ion leve ls a re high);

3. The potable water needs are below

200

m

3

per

day;

4.

The

land i s avai lable

for solar

s t i l l

s i tes ;

and

5. Such land i s in i sola ted

locations where other

sources of

energy

are non-exis tent .


40/155

21

Country Loc.ation

Year

,,

Feed Cover RCIIl Drke

Australia Huresl

1

196)

372

Brackish l au Rebuil

t

Huresk l

1966

)72

Brackish Class

Operating

Coaber

Pedy

1966

)160

Bnui5h

Class

Operating

Caiguna

1966

372

Bracki . b

Clau Operating

Hamelin Pool

1966

557

Brackish

Clan Operac.in&

Griffit.h

J ~ 7

413 Buck.iah Class

Operat.ing

Cape

Verde

Santa Y..aria

1965

743

Seilv.ater

Pl.astic

1 1 Santoll Haria

1968

Abandoned

Chile

Las 5aliDalO

1872 4460

Brad.ish Class

Ab.mdoned

QuillaE'ua

1968 1 S ~ l J a t e r

Clan Operating

Greece SyDi I 1964 2686 Sea1Jater

Planic Rebuil t

Symi I l

1968

2600 Se.vater

StT. Plas. Disn-..antled

Aegic.a. I 1965 1490 Sea\r.lter

J. lastic

Rebuilt

Aegina 11 1968 10486 Se31Jat er St.r. PIa&:. Abandoned

Salamis 1965 388

Se.auat.er

Plastic

Abandoned

Patmos 1967

8600

5uuater

Class

OperOlting

Kimolos 1968

2508

5eawater

Clau Operating

lHsyros

1969 2 5

5uv.ater

Class

Operating

Fiskardo 1971

22

Se0l10l3t.cr

Class

Operating

Kionioc

1971 2400

Seav.lt.er

C l . a s ~ Operating

Hegist i 1973 2528

S e ~ l 1 . : a t e r Class Operating

India Bhavnagar 1965 377

Se.avater

C ass

Operaticg

A\',rania

1978

1866

Bru.kish Class Operating

Mexico

Natividad Is l

1969 95

Seavatcr Class Operating

Pakistan

C\.7adar

1969 306

Sea\.73ter

0011;,

Operating

G'uadar

1I

1972

9072

Seavater

C l a s ~

Operating

Spain

Las Marinas

1966

868

Se.Jnlater Glass Operating

Tunis ia

Chakmou

1967

Brackish

Class

Operating

l- .ahd

ia

1968

13

Brackish Class Operating

U.

S.A.

Daycona Bead> 1959 228 Se.avater

Cb.sl;

Rebuilt

Daytona

B z :::h

1961 246

Seavater Class Disoantled

.Daytoo.a

Beach

1961 216

Sem.:-ater

Plas t i c

DiSI:iantled

Daytona

Beach

1963

148 Se.avate:r

Ph st ic

Dismantled

USSR

Bakb.arden

1969

600

Brackisb

Class Operating

\.Jest

Indies Pot i t 1967

171 Seavater Plast ic

Operating

St.Vincent

Rai t i

19.9

223

Seavater

Clau

Operating

hldia

Bilra

1980

Brackisb

Class

Gperot.ing

(capacity

2000

l /day)

Kult: .i,

198

Bracitisb Class

Operating

(c:apaci ty

:;000

l/da} )

China

\..'uzhi

j 9 ) ~

385

Se.al.1ater Class

Operat.ing

Zbungjian

1979

50

Sea ater

Class

Operating

TABLE

2.1

D T ON

THE MOST

PLANTS T F ~ T H VE

After 29)

IMPORT NT

SOL R

BEEN BUILT FROM

DISTILLATION

1872 TO

1980


41/155

22

I t h s also been found

tha t the

number

of

variables

inf luencing

the

produc t i v i t y o f s o l a r stills i s very h igh and t hey a re

o f t e n

independent. Among

t he

most impor tant are:

atmospheric

variab les

so lar

radiat ion,

wind

velocity

nd

ra infal l ) ;

design brine

depth,

insu la t ion , vapour

t igh tness , cons t ruc t ion

mater ia ls , maintenance)

nd operational techniques.

2.4 .1

f fec ts o f Atln::>s[tleri c Paraneters

1 . Ambient temperature:

Solar

s t i l l product iv i ty

increases s l igh t ly

as ambient a i r t empera ture increases . For each lOoF r i s e

in

ambient t empera ture ,

the

magni tude o f the

output i nc rease

averages 5 .

2.

Solar

r ad ia t ion :

As

f a r as the

atmospher ic

va r i ab le s

are

concerned,

the

solar s t i l l

product iv i ty

depends

almost ent i re ly

upon the

s o l a r

r ad ia t ion

in tens i ty .

It

w i l l

depend

to

some

extent

upon how

the

radiat ion i s dis t r ibuted throughout the day;

but it i s usual ly

suff ic ient

to consider

only

the to ta l rad ia t ion

received

each

day.

3.

Thermal

capac i ty : The thermal capac i t y o f a still has a smal l

ef fec t

on

i t s

performance.

4. Wind velOCity: As long as t he

still

is w e l l

sea led

to prevent

v p o ~ r leakage,

produc t i v i t y

i s

s l i g h t l y a f fec t ed

by wind.

However, if t he

product iv i ty to

a

still

is poor ly

sea led , wind

can lower

t he

great

extent.

5.

Rainfa l l :

S t i l l

produc t i v i t y c:;an

be

increased

by ca tch ing t he

ra infa l l .

2.4.2

Design f fec ts

1.

Brine-depth:

I t

h s been

concluded

tha t

the

shal lower

the

brine

depth o f a still t he higher t he t o t a l d a i l y product iv i ty , but


42/155

23

the

small water depth requires a=ura te levell ing

o f the

absorber

surface , s ince any

humps

in

the surface could cause

dry spots

thus

decreasing

the water surface area available for evaporation

as well as deterioration

of

the s t i l l .

2.

Materials

of

oonstruction:

s fa r as

construction i s

ooncerned,

useful information on materials i s yielded, part icular ly cover,

absorber and

sealants.

The t ransparen t cover

can

be e i t he r

glass or p las t i c but

glass

i s prefe rred to plas t i c because o f ts high t ransmiss ivi t :y

for

s o l a r

r a d i a t i on low t r a n s m i s s i v i t y for low temperature

rad ia t ion

high

we t t a b i l i t y

for

water

and

re l a t ive ly

high

s tabi l i ty of properties

over

a

long

period of

time.

The absorber

must

absorb so la r r ad ia t ion read i ly must be

water t ight and

shoUld

be

capable

of supporting high temperatures

without deleterious effects .

To prevent

vapour

leaks, cover

sea l ing i s most

important s ince

leakage can dramatically

decrease

the

production rate.

The Office

o f

Sal ine Water USA)

in the

r epor t [7] l i s t ed st ll component

materials

that

have proved t be reasonably sat i sfactory in

solar

s t i l l s

around the

world,

see

Table

2.2.

The

mater ia l s

are

l i s ted

in order of preference from a durabi l i ty standpoint).

3.

Insulation:

To ra ise

br ine temperature and reduce

heat

losses,

the

bottom and s ides o f

the st ll should be

insula ted. In some

cases, the annual productivity of an insulated s t i l l

i s 15

over

the uninsulated version.

4. Cover

design: The most practicable

cover for

large ins ta l la t ions

i s a glass se t a t

an

angle

of

10-20 degrees from the oorizontal.

The cover should

also

be placed a t no greater

distance

above the

br ine surface.

When a long l i f e up

to

a

maximum

of 5 years)

i n s t a l l a t i on i s

envisaged, o r in

i so la ted

locat ions and

where

glass t ranspor ta t ion could be d i f f i c u l t and expensive, glass

covers

can be

replaced

by p las t i c

ones

which should

be

t rea ted

for wet tabi l i ty to

prevent dropwise

oondensation.


43/155

Component

BJ

.

in

liner

Support

slruc.:ture

. Distillate

trough

Sealant

Piping and .valves

\ . ala 1 r a g e

rcscrvtlirs

24

Materials

Butyl

rubber

(OOI}-()OJO-in.

thick):

B:o;ph:llt

mats (O12-0lS-in. thick): black

p o l y e l h y l e n ~

O. ) )8in.

thick): roofing OlSph:l1t (o er con

crele. etc.)

Window

sla

(0-10

or

OI2-in. thick); cttablc

Tcdlar plastic IO-OO4-in.

thick)

Concrete:

concrete

block: aluminum: galvanized

metal:

redwood-

StainJess steel: butyl rubber (lining): black poly

. t:thylcne (lining)

Silicone rubber: asphalt caulking

compound:

butyl

rubber extrusions

PVC (polyvinylchloride): asbestos cement (for

saline water):

SS

(acrylonitrilc-butadicnc

slyrene)

Concreh:: masonry

Rl. lalively

short lifetimes.

TABLE.2.2:

LIST OF THE STILL COMPONENTS

THAT

HAVE

PROVED TO

BE REASONABLY SATISFACTORY

IN

SOLAR STILLS

AROUND THE

WORLD

f ter

(7)


44/155

5

5. Condensate

leakage:

The

leakage of the

condensate

from troughs

i s another

reason

for

the decrease of

still output . To prevent

the oondensate spi l l ing or overflowing, the troughs must

be

deep

and narrow

ElO IUgh to

minimise shaoowing of the brine.

2 4 3

Opera t ia la l

Techniques

t has been concluded t ha t

a long

term opera t ion

o f s t i l l s does

not

r qu i r

clearu.ng

in the

case of glass covers. However

plast ic covers

which

a t t rac t dust because

of the i r

eleclLostatic properties,

have

to

be

washed periodically. I t has been recommended t ha t a solar s t i l l

should

operate

cont inuously

throughout

the

year

and

feed water

preheat ing and

f lushing

methods can

be

used.

2 5


45/155

26

P

= annual i n t e res t

and

amor t iza t ion r a t e (percentage o f

investment)

MR =

annual maintenance

and

repa i r ,

labour and mater ia l s

costs

(percentage of

investment)

TI

=

annual

taxes and insurance charges (percentage o f

investment)

L

= annual operating

labour

costs

W

=

operating labour wages, $/man hours

Y

D

=

annual

unit yield

of dis t i l l ed water (gallons/m

2

)

Y

R

= annual unit yield

of =l lec ted

rainwater (gallons/m2)

n = area

of d i s t i l l e r

on which d i s t i l l a t e yie ld i s

based

m

2

)

AR = area of dis t i l le r on

which

ra infa l l

=l lec t ion

i s based

m

2

)

S = t o t a l cos t ( f ixed and operat ing) o f s a l t water supply

($/1000 gallons of product).

1.

P I Cooper:

The m ~ x m u m eff ic iency of s ing le e f fec t so la r

s t i l l s ,

Solar

Energy,

Vol

15, pp 205-217 (1973).

2. Maria

Telkes:

Fresh water from sea water by solar dist i l lat ion ,

Indust r ia l

and

Engineering Chemistry,

45 (5) pp

1108-1114 May

1953).

3. Maria Telkes:

Solar

s t i l l s ,

Proceedings of World Symposium on Applied Solar

Energy, Phoenix, Arizona,

pp 73-79

(November

1955).

4. Daniel Farrington:

Direct use

of the

sun 's

energy ,

Yale

Univers i ty

Press,

New

Haven,

374 pages

(1964)

[Chapter

10, 'Dist i l lat ion of

Water', pp

167-195].


46/155

27

5. Everett Howe:

Review o f still

t ypes ,

Chap t e r prepa red

fo r

U S o l a r

Dist i l la t ion Panel Meeting,

34

pages

October 14-18, 1968).

6.

Solar d i s t i l l a t i o n as

a

means o f meet ing sma l l - s ca le

water

demands , United Nations Publication (1970).

7. S G

Talbert;

J A

Eibling

and G G

Lof:

Manual of

so la r

d is t i l l a t ion of

sal ine

water , Office of Saline

Water, US Department

of

the

Inter ior ,

Res

and

Dev,

report

No

546

(1970).

8. M A S Malik, G N

Tiwari,

A Kumar

and

M S

Sodha:

Solar dist i l la t ion , Pergamon

Press,

Oxford, England (1982).

9.

Josiah

Harding:

Appara tus

f o r

s o l a r

d i s t i l l a t i o n ,

Proceed ings

o f t he

Inst i tut ion

of

Civil Engineers,

Vol 73, pp

284-288

(1883).

10.

Maria

Telkes:

Solar d i s t i l l e r

for

l i f e raf t s , US Off ice o f Science,

Report

No

525,

PB

21120, 24 pages

(19 June, 1945).

11.

C

Gcrnella:

Contribution

a

l 'e tude

de

l a d i s t i l l a t ion

so la i re

les

resu l ta ts

i ndus t r i e l s acquis en Alger ie apercu

s u r

l ' importance de

l ene r g i e t he rmique , Colloques in te rna t ionaux du Centre

National de

la

Recherche Scient i f ique [Applications thermiques

de

l ' energ ie

s o l a i r e dans le domaine de l a recherche e t de

l ' indus t r ie]

France,

pp 601-620 (1961).


47/155

28

12.

Savornin

and G Lejeune:

Etude

su r

1 ' evaporat ion e t

l a

condensat ion de 1 ' eau dans

1es

d is t i11a teu rs

sola i res , Co11oques

in te rna t ionaux

u cen t r e

National

de la Recherche Scient i f ique [Applications

thermiques

de

l ' energ ie so la i r e

dans l e domaine

de

l a

recherche

e t de

l ' industr ie] France, pp 589-600

(1961).

13. R N M:>rse and W R W Read:

A r a t iona l bas is for

the

engineer ing development o f a s o l a r

s t i l l , Solar Energy, Vol 12, pp

5-17

(1968).

14. P I Cooper:

The maximum

e f f i c iency o f s ing le -e f fec t

solar

s t i l l s , Solar

Energy,

Vol 15,

pp 205-217 (1973).

15. P I

ooper

and J A Appleyard:

The

construction and performance

of a

th ree effec t , wick

type,

t i l t e d so la r s t i l l , Sun a t

Work,

Vo1 12

(1),

pp

4-8

( f i r s t

quar te r , 1967).

16.

R W M:>rse:

The c ons t ruc t ion

and i n s t a l l a t i o n o f s o l a r st lls in

Australia ,

Desa1ination, vo1 5,

pp

82-89 (1968).

17. P I ooper and W R W Read:

Design

philosophy and operating

experience

for Austral ian

solar

s t i l l s ,

Solar

Energy, Vo1 16,

pp

1-8 (974).

18. German Frick and Jul io Hirschmann:

Theory and exper ience

with

s o l a r st lls in

Chile ,

Solar

Energy,

Vol

14, pp 405-413,

(1973).

19. M M Hafez and M K

Elnesh:.

Deminera l iza t ion o f sa l ine

wate r by

s o l a r

rad ia t ion in the

United

Arab

Republic ,

UN

Conference

on

New

Sources

of

Energy,

Paper 35/S/63,

Rome

10 pages (August

1961).


48/155

29

20. A Delyannis

and

E Piperoglou:

The

Patmos s o l a r

d i s t i l l a t i o n

plant , Technical

paper, Solar

Energy, Vol

12,

pp

113-115

(1968)

21.

M L

hanna

and

K N

Mathur:

Experiments

on deminera l i za t ion

of

water

in

North

India , UN

Conference on

New

Sources

of Energy, paper 35/S/115,

Rome

11

pages (August 1961).

22.

S D Gomkale S Y

Ahmed

R L Datta

and

D S

Datar:

Fresh

water from sea by so la r s t i l l , Paper presented a t the

Annual Meeting o f

the

Indian

In s t i t u t e

o f Olemical Engineers,

Bangalore,

India (Dec

1964).

23

S

Y

Ahmad S D

Gcmkale,

R

L

Datta

and

D S

Datar:

Scope and

development

of solar s t i l l s for

water desal inat ion

n

India ,

Desalination,

Vo1

5, pp

64-74

(1968).

24. Tunisian Atonic Energy

Ccmnission:

Report

o f

a c t i v i t i e s

1966-67,

Chapter

10,

Solar

energy ,

pp

53-76,

Solar

Dist i l la t ion,

pp

54-64,

n French (1967).

25.

Tunisian

Atonic

Energy Ccmnission:

Brochures descr ib ing so la r d i s t i l l a t i o n s t a t i ons

a t

Olibou,

Chekmou and

Mahdia, Tunisia ( in French

and

Arabic) ,

8

pages

each (1968).

26.

E D

Howe

and B W Tleimat:

Twenty

years

of

work

on

solar

di s t i l l a t ion

a t

the University of

California ,

Solar

Energy Vol 16, pp 97-195 (1974).

27.

Bat t e l l e Memorial Ins t i tu te : J

W

Bloemer, J R I rwin and J A

Eibling:

Final th ree years

progress

on s tudy and f i e ld evalua t ion o f

so la r

sea water

s t i l l s ,

June

1965,

OSW

Report

No

190

87

pages,

(May 1966).


49/155

3

8

V Batnn and R

Bairarrov:

Prospects

o f so la r st lls in Turkmenia ,

Solar

Energy,

Vol

16

(1), pp

38-40

(1966).

29. Delyarmis and

E

Delyarmis:

Sola r d i s t i l l a t i o n p lan t o f high capaci ty , Proceedings o f

Fourth

International

Symposium on Fresh Water from

the

Sea,

Vol

4, p

487

(1973).


50/155

31

QlAPl ER 3

SOL R DISTlLIATICN GENER L llIEDRY

3 .1 INl RCIXCl ICN

The bas ic p r incfp les

of opera t ion o f

s o l a r s t i l l s have been s t a t e d

and

developed by

many authors

[1

2

3 4]

to

the point

where

the

numerical signiUcance of the various

parameters

may be determined in

re la t ion

to

performance

and

fresh

water

production.

The work

which

was developed by

Dunkle [1]

in 1961

and

which

reviewed some o f

the

work on roof type s o l a r s t i l l s analysed

and

discussed the heat

and

mass t ransfer relat ionships and indicated the

ef fec t of temperature

and

pressure

on

the performance. That work

was

s l igh t ly modif ied by Morse and Read [2] in 1968 who considered the

heat and mass t ransfer relat ionships which

govern

the operation of a

so la r still

in

the unsteady s t a t e and

expressed

the

var ious

hea t

fluxes as

functions of the

= v e r

temperature.

The analysis

was

then

used to find the effects

on

output of changes in various

parameters

such as wind velOCity

ambient

temperature and heat

loss

from

the

base.

From t ha t

work

Cooper and Read [3] s tud ied both t heore t i ca l ly

and

pra c t i c a l l y the opera t ion

of

a

so la r

s t i l l .

They

showed t ha t

success fu l

development

of s o l a r

stills i s

dependent upon

a desfgn

ph i lo sophy i nvo l v i ng a

working

knowledge of t he

the rmal

character i s t ics of solar s t i l l operation. The design philosophy led

to

t h e establishment

of thermal

and

oost i e r i a for the selection

of

materials

and

design

of

component parts .

Final ly Malik e t a l

[4] in

t he i r

l a t e s t book on so la r d i s t i l l a t i o n

reviewed

the

work

which had

been

carr ied

out

up

to

that

time.


51/155

3

/ - o .q

T

s

Cwg

FIGURE 3 .1 : DIAGRAMMATIC SECTIONS

OF

SOLAR

STILL

SHOWTNG SIGNIFICANT ENERGY

TRANSPORT

STREAMS

TO

FROM

AND

WITHIN

THE

STILL


52/155

33

3.2

THEDRY

The relat ionships

which

govern the operation

of

a

so la r s t i l l

in

the

steady

s ta te condition, are the heat and

mass

t rans fe r ra te and the

energy

balances.

A diagrammat ic

cross - sec t ion

o f

a solar still

on

which

are indicated

the

heat and energy

f luxes

and the i r direct ions,

i s

shown in

Figure

3.1.

It

was

demonstrated t ha t a

se t

of e i gh t equat ions suf f i ces

to

describe the system.

3.2.1

Heat Balance

en

t he

Absorber and

Caller Assembly

The

energy balance

for

the

s t i l l

requires

tha t

th

to ta l solar energy

absorbed must

be equal

to the energy t ransferred from the CtJ


53/155

4

d

w

wb dt:

energy s to r ed

in

the system as the water temperature Tw

changes with t ime t ;

energy s to r ed

in

the cover

as

the cover temperature Tg

changes with t ime t

This

term can

be

neglected.

3 2 2 Heat

Balarx e

en the Als n ber

The hea t balance on

the

bas in which

was

given by

Morse and

Read [2]

i s as

follows:

d

< w

=

qlo

wb d qr

where i s

the

heat flux

by

evaporation

and condensation

qr

i s the heat

flux

by

radiation, and

i s

the

heat flux

by convection.

All the

terms are expressed in

SI uni ts

3 2 3 Heat Balarx e en the

Cover

(3.2)

he heat

t ransfer

between

the er

and the

s a l twa te r i s the

sum of

q r

qc qe

while the hea t flow to the su=oundings

i s

th i s to ta l

heat flux,

plus

,the

so la r

energy absorbed

by the

glass:

+

(3.3)

3 2 4 Heat Flux by Radia t i cn Ir

The

hea t

f lux by rad ia t ion

between

the cover and

the

water surface

was

given by

Dunkle

[1] and i s equal

in

SI system to:


54/155

35

(3.4)

where

EW i s the

emissivi ty of the

water

surface

and i s

usual ly taken

as 0.9

i s the Stefan-BoI zmann

constant

and i s equa l

to

5.6697 10-

8

m

1


55/155

36

where

h

lo

i s equivalent

to

heat t r an s f e r

coe f f i c ien t

base to the

surroundings

and

ground. This quant i ty i s

d i f f i cu l t

to

est imate because the temperature of

the

ground i s general ly

l.Il lkrn-m.

ArDther equation

which relates

the

heat dissipation from

the

oover

to

the ambient temperature Ta can be

added to

describe

the

system more

precisely.

This equation which was given by ooper and

Read

[3]

n SI

un ts i s :

3.8)

where

Ts

i s t he

sky temperature

and i s

equal, Sayigh

[5]

to:

3.9)

and

hga i s the convect ive heat t r ans fe r

coeff ic ien t

which i s

dependent on wind velocity and a= r d i ng to MacAdarns [6] ~ i s equal

to:

~

; 5.7

+

3.8

w

3.10)

where w

which

i s the a i r velocity, i s 0

i\

I

. < - ~ . -,

,

. ,

I'; - \ \ \ , , ~

o

40

60

80

100 120 140

I

COVEr

TMPRATUR . 1 9

180

FIGURE 3.4: CHARACTERISTIC

CHART FOR T H E R ~ L

PERFOR-

MANCE OF A SOLAR

STILL


58/155

39

From equat ion 3.6 and us ing

the

l a t en t

heat o f vapor iza t ion ,

the

evaporation

mass t ransfer

ra te which was given by

ooper

and

Read [3]

i s :

(3 .11)

3.3

REFEREN ES

1. R V Dunkle:

Solar water d i s t i l l a t i o n : the

roof- type

st ll and a mul t ip le

e f f e c t

d i f fus ion

s t i l l . In te rna t iona l Developments

i n

Heat

Transfer,

ASME,

pp

895-902

(1961).

2.

R N M >rse

and

W R W

Read:

A ra t iona l

bas i s fo r

t he engineer ing

development o f a s o l a r

s t i l l .

Solar Energy, Vol.

12,

pp 5-18

(1968).

3.

P I

Cooper and

W R W

Read:

Design

philosophy and operating experience for Austral ian

solar

s t i l l s .

Solar Energy,

Vol. 16,

pp 1-8

(1974).

4.

M A S Malik, G N

Tiwari,

A Kumar

and

M S Sodha:

Solar

dis t i l l a t ion .

pergamon Press, Oxford, England

(1982).

5. A A M Sayigh:

Solar energy engineering . Academic Press Inc., New YO:r:K (1977,

[pp 431-464

by E D

Howe and

B W T1eimat

' fundamentals o f wate r

desa l ina t ion ' ] .

6.

William

H MacAdams

Heat transmission . McGraw-Hill

Inc. ,

p 249 (1954).


59/155

4

rn PrER

4

A FllliE FLCM FLAT PLATE

SOLAR CXlLLEx IDR

llIEOREl ICAL

MDEL

4.1

l:Nl RCIXCl ICN

In

orde r

to

maximise the heat

absorption the = l l e c t o r

i s

incl ined.

The f l u i d

which

i s pumped

to

the

top

of the still f lows

f r ee ly

downward

n a t h in

f i lm and

subsequently the heated

surface

ra i ses

the

moving f lu id t empera ture and

evapora t ion

w i l l s t a r t . The

evaporated water which i s condensed on the

inner

s ide of the

=ver

i s

recovered

in a

s imi l a r

arrangement

to the bas in still. Such a

co l l ec t o r

i s

shown

in

Figure

4.1.

Depending

on

the

in tended

appl icat ion of the t i l t e d

= l l e c t o r under =nsidera t ion

[1-7]

many

theore t ica l analyses have been reported invest igat ing the f luid flow

and heat

t ransfer

character i s t ics of t h i s type [2 3 4 5 7].

An

ea r l y work by Col l i e r [2] cons idered

a descending

f l u i d

in

an

open f l a t

plate solar

= l l e c t o r to be

used

n a

refr igerat ion

system.

From his analysis which =mmenced

from

the energy equation n which

it

was assumed that the

flow

was steady he

developed

an expression

fo r

the

vapour

mass

f low and showed

t ha t the

performance

o f

the

=l l e c t o r was

in i t i a l ly

dependent

on environmental

=ndi t ions.

A s imi la r type o f study n

which Peng

and Hawell [3] endeavoured

to

improve

the

accuracy of the previous work was based on the

mass

and

heat balance equat ions . In order

to

obta in the tempera ture

dist r ibut ion

along

the

=l l e c to r

the

authors

assumed

the

flow

ra te

and

hea t

capac i ty o f

the

f lu id to

be

constant .

These

are

inva l id

assumpt ions s ince the evapora t ion could

be

cons i de r ab l e and

fur thermore the tempera ture change in the f lu id

could

be l a rge

mainly

for

a long. l a rge

plan t .

Johannsen

and Grossman

[4]

carr ied

out a study on a regenerating type

s o l a r co l l e c t o r

for

an a i r -condi t ion ing

system.

Sta r t ing

from

the

mass and hea t balance

equa t ions

they der ived

a genera l

formula

to

simulate the i r system.


60/155

\P

\ \


61/155

42

Aoc>ther invest igat ion

relevant to the

present

work

was presented by

Vaxman

and Sokolov [5]. The autlxlrs star ted the i r analysis from the

energy equation for both the

f lu id

f i lm and the black pla t e

neglecting evaporation

ra te and

assuming

steady

s ta te

ful ly

developed

f low

a free flow flat plate solar still (boutebila 1991)

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